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diff --git a/src/include/fe_utils/conditional.h b/src/include/fe_utils/conditional.h
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+/*-------------------------------------------------------------------------
+ * A stack of automaton states to handle nested conditionals.
+ *
+ * This file describes a stack of automaton states which
+ * allow a manage nested conditionals.
+ *
+ * It is used by:
+ * - "psql" interpreter for handling \if ... \endif
+ * - "pgbench" interpreter for handling \if ... \endif
+ * - "pgbench" syntax checker to test for proper nesting
+ *
+ * The stack holds the state of enclosing conditionals (are we in
+ * a true branch? in a false branch? have we already encountered
+ * a true branch?) so that the interpreter knows whether to execute
+ * code and whether to evaluate conditions.
+ *
+ * Copyright (c) 2000-2020, PostgreSQL Global Development Group
+ *
+ * src/include/fe_utils/conditional.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef CONDITIONAL_H
+#define CONDITIONAL_H
+
+/*
+ * Possible states of a single level of \if block.
+ */
+typedef enum ifState
+{
+	IFSTATE_NONE = 0,			/* not currently in an \if block */
+	IFSTATE_TRUE,				/* currently in an \if or \elif that is true
+								 * and all parent branches (if any) are true */
+	IFSTATE_FALSE,				/* currently in an \if or \elif that is false
+								 * but no true branch has yet been seen, and
+								 * all parent branches (if any) are true */
+	IFSTATE_IGNORED,			/* currently in an \elif that follows a true
+								 * branch, or the whole \if is a child of a
+								 * false parent branch */
+	IFSTATE_ELSE_TRUE,			/* currently in an \else that is true and all
+								 * parent branches (if any) are true */
+	IFSTATE_ELSE_FALSE			/* currently in an \else that is false or
+								 * ignored */
+} ifState;
+
+/*
+ * The state of nested \ifs is stored in a stack.
+ *
+ * query_len is used to determine what accumulated text to throw away at the
+ * end of an inactive branch.  (We could, perhaps, teach the lexer to not add
+ * stuff to the query buffer in the first place when inside an inactive branch;
+ * but that would be very invasive.)  We also need to save and restore the
+ * lexer's parenthesis nesting depth when throwing away text.  (We don't need
+ * to save and restore any of its other state, such as comment nesting depth,
+ * because a backslash command could never appear inside a comment or SQL
+ * literal.)
+ */
+typedef struct IfStackElem
+{
+	ifState		if_state;		/* current state, see enum above */
+	int			query_len;		/* length of query_buf at last branch start */
+	int			paren_depth;	/* parenthesis depth at last branch start */
+	struct IfStackElem *next;	/* next surrounding \if, if any */
+} IfStackElem;
+
+typedef struct ConditionalStackData
+{
+	IfStackElem *head;
+}			ConditionalStackData;
+
+typedef struct ConditionalStackData *ConditionalStack;
+
+
+extern ConditionalStack conditional_stack_create(void);
+
+extern void conditional_stack_destroy(ConditionalStack cstack);
+
+extern int	conditional_stack_depth(ConditionalStack cstack);
+
+extern void conditional_stack_push(ConditionalStack cstack, ifState new_state);
+
+extern bool conditional_stack_pop(ConditionalStack cstack);
+
+extern ifState conditional_stack_peek(ConditionalStack cstack);
+
+extern bool conditional_stack_poke(ConditionalStack cstack, ifState new_state);
+
+extern bool conditional_stack_empty(ConditionalStack cstack);
+
+extern bool conditional_active(ConditionalStack cstack);
+
+extern void conditional_stack_set_query_len(ConditionalStack cstack, int len);
+
+extern int	conditional_stack_get_query_len(ConditionalStack cstack);
+
+extern void conditional_stack_set_paren_depth(ConditionalStack cstack, int depth);
+
+extern int	conditional_stack_get_paren_depth(ConditionalStack cstack);
+
+#endif							/* CONDITIONAL_H */